New study finds a link between smog and birth weight — maybe

A new international study is suggesting that pregnant women exposed to smog have a greater risk of having a baby with low birth weight.

Researchers led jointly by Tracey J. Woodruff, professor of obstetrics and gynecology and reproductive sciences at University of California San Francisco and Jennifer Parker, of the National Center for Health Statistics at the US Centers for Disease Control and Prevention, published their findings this week in Environmental Health Perspectives.

The study, the largest of its kind, analyzed data collected from more than three million births in nine nations at 14 sites in North America, South America, Europe, Asia and Australia. The researchers found that the higher the pollution rate, the greater the rate of low birth weight (below 2.5 kg) all over the world.

The investigators noted in their background information that while a growing body of evidence has linked maternal exposure to outdoor air pollution with adverse effects on fetal growth, the data have been inconsistent.

“This is a perfect example of everything not to do in a study,” says ACSH’s Dr. Gilbert Ross. “Researchers neglected to mention how the particulate exposures were measured in all the countries. Did they use national levels? Did they use computer modeling, which is often fudged?”

Dr. Ruth Kava adds, “This is an ecological study, so the data from individuals can’t be separated out and controlled for confounding influences. It’s the type of study that helps form new hypotheses, but certainly cannot show a causal link.”